Optical memory devices using a silicon wire grid polarizer and methods of making and using

ABSTRACT

Long term optical memory includes a storage medium composed from an array of silicon nanoridges positioned onto the fused silica glass. The array has first and second polarization contrast corresponding to different phase of silicon. The first polarization contrast results from amorphous phase of silicon and the second polarization contrast results from crystalline phase of silicon. The first and second polarization states are spatially distributed over plurality of localized data areas of the storage medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This present application is a divisional of U.S. patent application Ser.No. 16/461,781, filed May 16, 2019, which is the U.S. national stageapplication of PCT Application No. PCT/RU2016/000796, filed Nov. 18,2016, both of which are hereby incorporated by reference in theirentirety.

FIELD

The invention relates to data storage devices including optical discswhich may store the data for very long time. The invention also relatesto wire grid polarizers that have a grid of conductors located on thesurface of a substrate. The invention also relates to methods anddevices for forming periodic wire grids with a period of, for example,90 nm or less.

BACKGROUND

In conventional long term optical memory, data is recorded in localizeddata areas of fused silica glass modified by femtosecond laser pulses.At least some optical memory uses disc having multiple layers ofmicroscopic data areas (dots) with differing refractive indices. In someinstances, the microscopic data areas contain nanogratings, whichinclude lamina structures embedded within the fused silica material. Thenanogratings allow denser data writing due to each nanograting beingcharacterized by individual orientation and retardance, which arecontrolled by polarization and intensity of writing laser beam. However,the size of data areas in one example is 3.7 μm, which limits the datadensity. For example, current compact discs have sub-micrometer dataareas. Many known memory discs of fused silica glass with imbeddedstructures are produced by an expensive femtosecond laser technique,have low data density, and require complicated microscope systems tocompensate for spherical aberrations and to extract data frommulti-layer arrays of dots.

Optical memory discs are known that use a sapphire substrate andcrystalline silicon film with localized data areas of silicon inamorphous phase formed by laser pulses. The principle of data storagefor these discs is based on the difference in light transmission andreflectance for amorphous and crystalline silicon phases. The phasechange of silicon is restorable using one laser pulse to transformcrystalline phase of silicon into an amorphous phase in the data area,whereas the other laser pulse with different power and pulse durationmay provide back transformation of the amorphous phase of silicon intothe crystalline phase in the same data area, thus resulting in writingand erasing the data. However, the difference in light transmission andreflectance between amorphous and crystalline phases in silicon layersdoes not exceed about 10 times and the size of localized data areas israther large at about 10 μm, which may limit the performance of theoptical memory based on local phase change of silicon film.

BRIEF SUMMARY

One embodiment of the present invention is an optical memory whichincludes an optically transparent substrate and a storage mediumdisposed on the substrate and including a substantially planar array ofsilicon nanoridges, where the silicon nanoridges are configured andarranged for transformation between a first state and a second state.The first and second states have different responses to polarized lightof at one or more wavelengths. The storage medium is configured andarranged to be spatially divided into a plurality of data areas.

Another embodiment is an optical memory device including the opticalmemory described above, a light source to produce a beam of polarizedlight directed onto the optical memory; and a detector to receive anddetect polarized light from the data areas of the optical memory inresponse to the beam of polarized light being directed onto the opticalmemory.

In at least some embodiments, the first state corresponds to anamorphous phase of silicon and the second state corresponds to acrystalline phase of silicon. In at least some embodiments, thedifferent responses to polarized light includes a polarization contrastratio of the array in the first state being measurably different from apolarization contrast ratio of the array in the second state. In atleast some embodiments, the optically transparent substrate is fusedsilica glass.

In at least some embodiments, the array of silicon nanoridges includes aquasi-periodic, anisotropic array of elongated ridge elements having awave-ordered structure. In at least some embodiments, a period of thearray of elongated ridge elements is in a range from 40 to 90 nm. In atleast some embodiments, a height of silicon nanoridges is in a range 130to 200 nm. In at least some embodiments, the silicon nanoridges areoriented along one rectilinear direction. In at least some embodiments,the nanoridges are oriented along a plurality of concentric circles. Inat least some embodiments, the nanoridges are oriented along a pluralityof radial rays.

In at least some embodiments, the light source is configured andarranged to produce the beam of polarized light having a wavelength in arange of 395-450 nm.

Yet another embodiment is a method of forming a hard nanomask on arotating substrate, the method including depositing a first material toform a surface layer on top of a substrate; providing a flux of ions ina form of a sector centered to a rotation center of the substrate;rotating the substrate under the flux of ions; and irradiating a surfaceof the surface layer with the flux of ions during the substrate rotationuntil a hard nanomask is formed. The nanomask includes a substantiallyperiodic array of elongated elements having a wavelike cross-section, atleast some of the elongated elements having the following structure incross-section: an inner region of the first material, a first outerregion of a second material covering a first portion of the innerregion, and a second outer region of the second material covering asecond portion of the inner region and connecting with the first outerregion at a wave crest, where the first outer region is substantiallythicker than the second outer region, and where the second material isformed by modifying the first material by the ion flow.

A further embodiment is a method of forming a hard nanomask with theelements oriented along the concentric circles centered to the substraterotation center. Such a nanomask is formed by the flux of ions havingprojection to the substrate surface along the central radial axis of thesector.

Another embodiment is a method of forming a hard nanomask with theelements oriented along the radial rays having common onset at thesubstrate rotation center. Such a nanomask is formed by the flux of ionshaving projection to the substrate surface perpendicular to the centralradial axis of the sector.

In at least some embodiments, a period of the substantially periodicarray is in a range from 40 to 90 nm. In at least some embodiments, thefirst material is silicon or amorphous silicon. In at least someembodiments, the flux of ions comprises a flux of N₂ ⁺, N⁺, NO⁺, NH_(m)⁺, or a mixture of a) Ar⁺ and N₂ ⁺, b) Kr⁺ and N₂ ⁺, or c) Xe⁺ and N₂ ⁺ions. In at least some embodiments, a thickness of the first outerregion is at least 4 nm. In at least some embodiments, a thickness ofthe second outer region is no more than 2 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a wire grid polarizer containingnanowires of amorphous silicon;

FIG. 2 is a graph of a dependence of the contrast ratio (CR) of anamorphous silicon nanowire polarizer, which was made using a WOSnanomask, on successive stepwise annealing at different temperatures(from room temperature to 700° C.), where each annealing temperaturestep was held for an hour then the polarizer was cooled to roomtemperature and CR value was measured at the wavelength of light 405 nm,according to the invention;

FIG. 3 is a graph of the extinction coefficient (k) versus wavelength oflight for amorphous silicon (a-Si) and crystalline silicon (c-Si)materials;

FIG. 4 is a perspective view of a small part of storage medium based onthe array of silicon nanowires to form a localized data area, accordingto the invention;

FIG. 5 is a SEM top view of a hard WOS nanomask formed by the ion fluxon the surface of amorphous silicon layer disposed on a fused silicaglass substrate;

FIG. 6 is a SEM cross-sectional view, angled at 82°, of an amorphoussilicon nanowire polarizer based on a WOS nanomask and composed of aplanar array of amorphous silicon nanoridges positioned onto a fusedsilica glass substrate, according to the invention;

FIG. 7 schematically illustrates steps in one embodiment of a method forformation of an amorphous silicon nanowire polarizer, such as a wiregrid polarizer, using a hard WOS nanomask formed in amorphous siliconlayer, according to the invention;

FIG. 8 is a perspective view of a small fragment of silicon nanowirestorage medium near two data areas, where the silicon nanowires have acrystalline phase, and corresponding dependence of transmittance (T_(S))of S-polarized light beam with diameter close to that of the data areason the distance along the line across the data areas, according to theinvention;

FIG. 9 schematically illustrates optical memory discs with arrays ofsilicon nanoridges, in which nanoridges have different possibleorientations: along one rectilinear direction, along concentric circles,and along radial rays, according to the invention;

FIG. 10 is a perspective view of a small fragment of an optical memorydisc containing two crossed amorphous silicon wire grid polarizers, oneof which has a data area, where the silicon nanowires have a crystallinephase, according to the invention;

FIG. 11 is a partial perspective view of the mutual orientation of anoptical memory disc with a storage medium composed from an array ofsilicon nanoridges oriented along concentric circles and a beam ofpolarized light focused onto the storage medium, according to theinvention;

FIG. 12 is a partial perspective view of the mutual orientation of anoptical memory disc with a storage medium composed from an array ofsilicon nanoridges oriented along radial rays and a beam of polarizedlight focused onto the storage medium, according to the invention;

FIG. 13 is a diagram illustrating a mutual arrangement of ion flux and asector diaphragm to form a circular WOS nanomask having elementsoriented along concentric circles on a disc surface, according to theinvention; and

FIG. 14 is a diagram illustrating a mutual arrangement of ion flux and asector diaphragm to form a radial WOS nanomask having elements orientedalong radial rays on a disc surface, according to the invention.

DETAILED DESCRIPTION

Detailed descriptions of the preferred embodiments are provided herein.It is to be understood, however, that the present inventions may beembodied in various forms. Therefore, specific implementations disclosedherein are not to be interpreted as limiting.

As described herein, a long term optical memory is based on a siliconWGP with microscopic data areas, in which the silicon phase is changedfrom crystalline to amorphous or vice versa, uses low cost lasers fordata writing, has sufficient data density in single surface layer, orallows microscope-free data readout or any combination of thesefeatures.

Arrays of amorphous silicon (a-Si) nanowires are used as wire gridpolarizers (WGP) for violet and ultra violet (UV) applications. Anexample of an a-Si WGP is shown in FIG. 1. On the plane surface of atransparent substrate 1 silicon nanowires 2 are disposed and orientedalong one direction, i.e. Y axis. For good WGP performance the nanowireperiod λ should be considerably smaller than the wavelength of light.The nanowire height (h) and width (w) affect the WGP performance. Linearpolarized light with P polarization, which is along X axis andperpendicular to the nanowires, mostly transmits through the WGP and canbe characterized by transmittance T_(P). Light with S polarization,which is along X axis and parallel to the nanowires, is mostly reflectedby WGP and only small amount of S polarized light as characterized bytransmittance T_(S) penetrates through WGP. The ratio of T_(P) to T_(S)is known as the polarizer contrast ratio CR=T_(P)/T_(S). Examples of theCR values of known WGP based on silicon nanowires are CR=90 for ananowire period of 140 nm and a light wavelength of 365 nm and CR=324for a nanowire period of 120 nm and a light wavelength of 394 nm.

A method for nanorelief formation on a film surface, utilizing plasmamodification of wave ordered structure (WOS) formed on amorphous siliconlayer, is disclosed in Russian Patent Application RU 2204179,incorporated herein by reference. Using a WOS nanomask, various WGP maybe fabricated as disclosed in U.S. Pat. No. 7,768,018 and in PCT PatentApplication Publication No. 2014/142700, both of which are incorporatedherein by reference. WOS-based WGP may include silicon nanowires ornanoridges disposed on the surface of optically transparent substrate asshown in structure 1023 of FIG. 8 in the description of U.S. Pat. No.7,768,018.

An array of silicon nanowires or nanoridges can be used as a durablestorage medium. The array is positioned onto a substrate, such as afused silica glass substrate. It has been found that the CR of a siliconWGP measured at the light wavelength of 405 nm is considerably affectedby annealing of the WGP. FIG. 2 is a graph of the dependence of CR of anamorphous silicon WGP, which was made using WOS nanomask, on successivestepwise annealing. Each annealing temperature step was held for an hourthen the WGP was cooled down to room temperature and the CR value wasmeasured at an optical test bench equipped with a 405-nm semiconductorlaser. High CR values in the range 500-15,000 were measured forWOS-based a-Si WGP samples with a nanoridge period λ=70 nm and differentnanoridge heights, h, in the corresponding range h=130-200 nm. Afterannealing at 700° C. the CR value of a WOS based a-Si WGP abruptly dropsfrom CR>10,000 down to CR≈10. It is thought that this considerablechange in CR may be explained by the annealing at 700° C. resulting in aphase transformation of amorphous silicon (a-Si) into crystallinesilicon (c-Si) material in the silicon nanowires of WGP. Known spectraldependences of extinction coefficient (k) for a-Si and c-Si materialsare shown in FIG. 3. The extinction coefficient k characterizes theabsorption of light by these materials. A prominent difference between kvalues for a-Si and c-Si at the wavelength of 405 nm may result insignificant enhancement of transmittance T_(S) for silicon WGP thusresulting in corresponding significant lowering of CR values. In atleast some embodiments, light having a wavelength in a range from 395 to450 nm, or 400 to 410 nm, can be used for reading or writing an opticalmemory formed using a WOS-based a-Si WGP.

An estimation of the storage time at room temperature for a storagemedium based on an a-Si WGP is about 56 million years. The estimation ismade on the basis of the activation energy value for the crystallizationprocess for silicon nanowires being equal to 1 eV and because siliconWGP withstands annealing at 650° C. for at least 2 hours without changein CR value. Thus, the storage medium can be considered as durable andsuitable for long term memory storage.

FIG. 4 shows a part of a storage medium composed from an array ofsilicon nanowires. Nanowires 2 are made of a-Si material whereasnanowires 3 contain c-Si material within a circular localized data area4. Although the data area 4 is illustrated as circular, in otherembodiments, other regular or irregular data areas can be usedincluding, but not limited to, square, rectangular, trapezoidal,triangular, pentagonal, hexagonal, octagonal, decagonal, or dodecagonalareas may be used. Outside the data area 4 the nanowires are in anamorphous state. In at least some embodiments, the data area 4 may havea diameter in a range from 0.1 to 5 micrometers; however, larger orsmaller data areas may also be used.

The data area 4 may be formed by a laser pulse, for example, atwavelength of about 405 nm with a radiation power in the 10-100 mW rangefocused on the area of 0.5-2 μm in diameter and pulse duration in therange 10⁻²-10⁻⁶ s. Other continuous wave (CW) or pulsed lasers known inthe art may be used to locally change the phase of silicon in a-Sinanowires. This phase change can be implemented partially, i.e. only apart of a-Si nanowire volume is transformed into crystalline phase bylaser irradiation within data area 4, thus reducing the power of thelaser beam used. This exemplifies a writing of data on the opticalmemory. Data can be erased by changing the crystalline phase of siliconto the amorphous phase by laser irradiation. Such an arrangement isdescribed in, for example, U.S. Pat. No. 4,556,524, incorporated hereinby reference.

Amorphous silicon (a-Si) WGP can be formed using a WOS nanomask in ana-Si layer deposited on an optically transparent substrate as disclosedin U.S. Pat. No. 7,768,018 and in PCT Patent Application Publication No.2014/142700, both of which are incorporated herein by reference. A WOSnanomask, with or without ordering, may be used. FIG. 5 shows a SEM topview of a hard WOS nanomask formed by the ion flux on the surface ofamorphous silicon layer deposited onto a fused silica glass substrate.Waves (elongated elements) of a WOS nanomask are mostly elongated in theY-axis direction. Each wave on one slope has a thick silicon nitrideregion 6 viewed as black in FIG. 5 and on the opposite slope has aregion 16 of thin silicon nitride viewed as white in FIG. 5. Elongatedelements of the nanomask are arranged into a wave-ordered structurepattern as viewed from the top. In this example, the average period ofthe WOS, λ, is approximately equal to 80 nm.

FIG. 6 shows a SEM cross-sectional view, angled at 82°, of a WOS baseda-Si WGP that is composed of a planar array of a-Si nanoridges 2positioned onto a fused silica glass substrate 1. An ultrathin goldlayer of about 10 nm was deposited onto the WGP sample for SEMobservation. In this example, the average period of the array, λ, isapproximately equal to 70 nm and the height of the nanoridges, h, isapproximately equal to 200 nm. This WGP has CR≈15,000 and T_(P)≈26%. Inat least some embodiments, the period of the array is in a range from 40to 90 nm.

FIG. 7 illustrates one embodiment of a method to manufacture a WOS-baseda-Si WGP on a transparent glass substrate. It shows a structure 701,including a substrate (e.g., fused silica glass) 1 and an amorphoussilicon (a-Si) layer 5 (for example, approximately 240-330 nm thick)disposed on the substrate.

The amorphous silicon (a-Si) layer 5 may be deposited, for example, bymagnetron sputtering of a silicon target, by silicon target evaporationwith an electron beam in high vacuum, or by any other method known inart. The thickness of the layer 5 is selected to enable the formation ofa nanostructure with a desired period, (for example, a period ofapproximately λ≈60-80 nm).

A WOS 7 is formed on the surface of the a-Si layer 5 which results inthe structure 702. In this example, the WOS serves as a nanomask (WOSnanomask) for etching of silicon. The WOS can be formed using an obliqueflow 17 of nitrogen N₂ ⁺ or other ions (for example, N⁺, NO⁺, NH_(m) ⁺,or a mixture of a) Ar⁺ and N₂ ⁺, b) Kr⁺ and N₂ ⁺, or c) Xe⁺ and N₂ ⁺ions). Each WOS wave (elongated element) in cross-section has awave-like shape and contains thick silicon nitride regions 6 and thinsilicon nitride regions 16, which are connected at a wave crest. Regions6 and 16 are both formed from silicon by the nitrogen ion beam. WOSelements are mostly elongated in one direction perpendicular to thedrawing plane of FIG. 7. The top view of this WOS pattern is similar tothat shown in FIG. 5. As shown in the structure 702, the thick siliconnitride regions 6 of the WOS are spaced from the surface of thetransparent substrate 1 by a distance D, which may range from, forexample, 110 to 180 nm.

Referring again to FIG. 7, after the WOS nanomask 7 is formed, thestructure 702 is modified by applying a reactive-ion plasma (Cl₂,Cl₂—Ar, HBr—O₂ or Cl₂—He—O₂ or by any other etching method known in art)to the amorphous silicon layer 5, using the WOS nanomask 7. The plasmaprocess results in silicon etching down to the surface of substrate 1,thus forming a WOS based a-Si WGP, which is shown as the structure 703.In at least some embodiments, the plasma process results in a modifiedWOS nanomask having silicon nitride regions 6 a formed on top ofnanoridges 2 of amorphous silicon, as shown in the structure 703 of FIG.7. The thickness of the regions 6 a may become thinner than thethickness of the original regions 6 during plasma etching.

FIG. 8 shows a small part of an a-Si WGP having two circular data areas4, where the silicon nanowires 3 have a crystalline phase. Outside dataareas 4 the nanowires 2 are of amorphous silicon. In at least someembodiments, the intensity pattern 9 is generated by S-polarized lightbeam focused onto the WGP plane to a diameter close to that of the dataareas, the beam traveling along the line 8 across the data areas. Adetector that measures the intensity pattern 9 may read the informationfrom the WGP encoded in data areas as known in the art. In at least someembodiments, for a corresponding change in CR from 1,000-15,000 outsidedata areas down to about 10 within data areas the ratio of maximum tominimum in intensity patterns may exceed 100-1,500. The CR differencefor WOS-based WGP is greater than the change in light transmittancebetween a-Si data areas and surrounding c-Si for known optical discsbased on silicon films. In general, the greater the difference inintensity pattern the better is the performance of optical memory.

FIG. 9 schematically illustrates several embodiments of optical memorydiscs with arrays of silicon nanoridges having different arrangements.In at least some embodiments, the optical memory disc 10 may haverectilinear orientation of nanoridges in the array. In at least someembodiments, optical memory disc 11 may have nanoridges oriented alongconcentric circles. In at least some other embodiments, optical memorydisk 12 may have nanoridges disposed along radial rays. In at least someembodiments, for readout of the disc 10, one may use a polarizationrotation device to keep the polarization direction of the light beamparallel to the nanoridges during disk rotation. Such polarizationrotating devices based on liquid crystal cells are known in the art (forexample, system for continuously rotating plane of polarized light isdisclosed in U.S. Pat. No. 5,412,500, incorporated herein by reference).Note that discs 11 and 12 may be read using the light beam having afixed polarization direction. In at least some embodiments, for discs 11and 12 a certain adjustment of the polarization direction of the readinglight beam may be performed with the use of a polarization rotatingdevice.

In at least some embodiments, the optical memory disc may be composedfrom two crossed a-Si WGP, one of which has circular localized dataareas 4, where the silicon nanowires 3 have crystalline phase, as shownin FIG. 10. Outside data areas 4 the silicon nanowires 2 are amorphous.For such embodiments, unpolarized light may be used to readout the dataareas.

FIG. 11 illustrates one embodiment, where the optical memory disc 11with an array of silicon nanoridges oriented along concentric circles isirradiated by polarized light beam 13 focused onto the array plane fordata readout, the beam S polarization being fixed along the circlesduring disc rotation or adjusted using polarization rotating device.

FIG. 12 illustrates another embodiment, where the optical memory disc 12with array of silicon nanoridges oriented along radial rays isirradiated by polarized light beam 13 focused onto the array plane fordata readout, the beam S polarization being fixed along the radial raysduring disc rotation or adjusted using polarization rotating device.

In at least some embodiments, a-Si nanoridges are oriented alongconcentric circles on a disc surface and fabricated using a circular WOSnanomask, which has wave-like elements mostly elongated along theconcentric circles of the disc. In at least some embodiments, a circularWOS nanomask is fabricated by a flux of ions having the shape of asector centered to the center of the circles. Such sector-shaped ionfluxes may be formed, for example, by sector diaphragms. FIG. 13illustrates a mutual arrangement of N₂ ⁺ ion flux 18 and a sectordiaphragm 14 with opened sector 15 to form a circular WOS nanomaskhaving elements 7 a oriented along concentric circles on the surface ofdisc 11 a. The disc surface is covered by an a-Si layer and irradiatedby the ion flux only within the opened sector 15 of angle φ centered toX axis, i.e. central radial axis of the sector. The ion flux is directedalong the vector 18 in the ion incidence plane XZ at angle θ to the discsurface normal (Z axis). During ion irradiation the disc is rotatedaround Z axis under the fixed diaphragm 14. In at least someembodiments, a circular WOS nanomask may be formed during a singlerevolution of the disc. In at least some embodiments, a circular WOSnanomask may be formed during a few revolutions of the disc. In at leastsome embodiments, the sector angle φ in diaphragm is of about 10° to20°. In at least some embodiments, the ion fluence for the formation ofcircular WOS nanomask is the same as the fluence used for the formationof other WOS nanomasks.

In at least some embodiments, a-Si nanoridges are oriented along radialrays on a disc surface and fabricated using a radial WOS nanomask, whichhas wave like elements mostly elongated along the radial rays of thedisc. FIG. 14 illustrates a mutual arrangement of N₂ ⁺ ion flux 19 and asector diaphragm 14 with opened sector 15 to form a radial WOS nanomaskhaving elements 7 b oriented along radial rays on the surface of disc 12a. The disc surface is covered by an a-Si layer and irradiated by theion flux only within the opened sector 15 of angle φ centered to X axis,i.e. central radial axis of the sector. The ion flux is directed alongthe vector 19 in the ion incidence plane YZ at angle θ to the discsurface normal (Z axis). During ion irradiation, the disc is rotatedaround Z axis under the fixed diaphragm 14. In at least someembodiments, a radial WOS nanomask may be formed during singlerevolution of the disc. In at least some embodiments, a radial WOSnanomask may be formed during several revolutions of the disc. In atleast some embodiments, the sector angle φ in diaphragm is of about 10°to 20°. In at least some embodiments, the ion fluence for the formationof radial WOS nanomask is the same as the fluence required for theformation of other WOS nanomasks.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method of forming a hard nanomask on arotating substrate, the method comprising: depositing a first materialto form a surface layer on top of a substrate; providing a flux of ionsin a form of a sector centered to a rotation center of the substrate;rotating the substrate under the flux of ions; and irradiating a surfaceof the surface layer with the flux of ions during the substrate rotationuntil a hard nanomask is formed, the hard nanomask comprising asubstantially periodic array of elongated elements having a wavelikecross-section, at least some of the elongated elements having thefollowing structure in cross-section: an inner region of the firstmaterial, a first outer region of a second material covering a firstportion of the inner region, and a second outer region of the secondmaterial covering a second portion of the inner region and connectingwith the first outer region at a wave crest, wherein the first outerregion is substantially thicker than the second outer region, andwherein the second material is formed by modifying the first material bythe flux of ions.
 2. The method of claim 1, wherein the flux of ions hasa projection to the surface of the surface layer along a central radialaxis of the sector and the elongated elements of the hard nanomask areoriented along concentric circles centered about the rotation center. 3.The method of claim 2, wherein the flux of ions has a projection to thesurface of the surface layer perpendicular to the central radial axis ofthe sector and elongated elements of the hard nanomask are orientedalong radial rays extending radially relative to the rotation center. 4.The method of claim 1, wherein a period of the substantially periodicarray is in a range from 40 to 90 nm.
 5. The method of claim 1, whereinthe first material is silicon or amorphous silicon.
 6. The method ofclaim 1, wherein the flux of ions comprises a flux of N₂ ⁺, N⁺, NO⁺,NH_(m) ⁺, or a mixture of a) Ar⁺ and N₂ ⁺, b) Kr⁺ and N₂ ⁺, or c) Xe⁺and N₂ ⁺ ions.
 7. The method of claim 1, wherein a thickness of thefirst outer region is at least 4 nm.
 8. The method of claim 1, wherein athickness of the second outer region is no more than 2 nm.